This invention relates to polyester molding compounds and additives therefor characterized by uniform pigmentability and dimensional stability.
For many years polyester resins based on an unsaturated polyester and an unsaturated monomer, such as styrene, have been used in the preparation of molding compositions. When formulated in a known manner with appropriate adjuvants such as fillers, pigments, curing agents, reinforcing agents, and the like, such polyester-based molding compositions may be used in conventional molding processes such as sheet molding or bulk molding to prepare a wide variety of materials or articles which are generally characterized by high strength, light weight and excellent chemical resistance. However, articles fabricated from such molding compositions by conventional molding techniques commonly exhibit poor dimensional stability, due in a large part to shrinkage during the molding process. The poor dimensional stability is typically evidenced by a rough or warped surface (high profile) and surface irregularities such as deep sink marks opposite structural ribs. Thus, considerable difficulties are encountered in the production of molded articles where close size and shape tolerance limits are required since in many instances an inordinate amount of labor is required after removal of a part from the mold to shape it to the exact finish, size or shape required for a given application.
Considerable effort has been expended in recent years, in the development of dimensionally stable (low-profile) polyester molding compositions. It is now well known to those skilled in the art that with the addition of thermoplastic polymers to polyester systems, such as polyester-styrene based resins, there may be prepared compositions which, when formulated with the appropriate adjuvants to form molding compounds, display minimal shrinkage or expansion during the molding process and may be formed with smooth surfaces (low-profile). The thermoplastic polymers which have been used for this purpose include, for example, such materials as polyvinyl acetate, cellulose acetate, cellulose butyrate, polymethyl methacrylate, polystyrene, polyethylene, polyvinyl chloride and saturated polyesters.
With the advent of low-profile unsaturated polyester systems, molding techniques have been increasingly employed in the fabrication of automotive parts such as hood scoops, fender extensions and a wide variety of other parts which require both a smooth surface and close adherence to the size and shape of a precision machined mold.
In the production of articles such as automotive parts, it is particularly desirable to impart the desired color to the articles by addition of a pigment to the molding compound prior to the molding process. This technique eliminates the need for an additional coating step after molding, and in general, minimizes maintenance and extends the useful life of the article since the color is dispersed throughout the material rather than only on the surface. However, it has been found that, in low profile unsaturated polyester molding compounds containing thermoplastic additives, the achievement of uniform coloration is difficult at best. The addition of a thermoplastic polymer, with the exception of polyethylene and polystyrene, to an unsaturated polyester molding compound, results in a non-uniform distribution of the pigment and a "phase out" or separation of non-pigmented and pigmented areas during molding or curing. The result is an undesirable mottled appearance of the surface. When polyethylene or polystyrene is employed as the thermoplastic polymer additive, considerable improvement in the uniformity of pigmentation is achieved. However, the dimensional stability, although improved by the addition of polyethylene or polystyrene is notably less than that achieved with other thermoplastic polymer additives, which phase out during molding or curing. Thus a manufacturer utilizing molding compounds which incorporate thermoplastic polymer additives must choose between a thermoplastic polymer which imparts good surface smoothness and dimensional stability, but which cannot be pigmented uniformly, and a thermoplastic polymer which imparts good pigmentability but is substantially poorer in surface smoothness and dimensional stability.
Accordingly, it is an object of the present invention to provide unsaturated polyester based molding compositions which exhibit a high degree of dimensional stability. It is a further object to provide low-profile additive compositions which may be added to unsaturated polyester based molding compositions to impart a high degree of dimensional stability thereto without adverse effect on the pigmentability of the composition. It is a still further object to provide a method for the production of molded articles from unsaturated polyester based molding compounds, wherein the molded articles are uniformly pigmented and are within close tolerance limits of the size and shape imparted by the mold. It is an additional object to provide articles of manufacture molded from novel polymeric compositions and having uniform pigmentation and closely reproducible size and shape.